Cultured muscle produced by mechanical conditioning

ABSTRACT

A method of producing organized skeletal muscle tissue from precursor muscle cells in vitro comprises: (a) providing precursor muscle cells on a support in a tissue media; then (b) cyclically stretching and relaxing the support at least twice along a first axis during a first time period; and then (c) optionally but preferably maintaining the support in a substantially static position during a second time period; and then (d) repeating steps (b) and (c) for a number of times sufficient to enhance the functionality of the tissue formed on the support and/or produce organized skeletal muscle tissue on the solid support from the precursor muscle cells.

RELATED APPLICATIONS

This application claims priority to and is a divisional of U.S. patentapplication Ser. No. 11/279,671, filed Apr. 13, 2006, and also claimsthe benefit of U.S. Provisional Patent Application Ser. No. 60/671,600,filed Apr. 15, 2005, the disclosure of each of which is incorporated byreference herein in its entirety.

FIELD OF THE INVENTION

The present invention concerns methods and apparatus for the growth ofskeletal muscle in vitro.

BACKGROUND OF THE INVENTION

Loss of functional skeletal muscle due to traumatic injury, tumorexcision, etc., produces a physiological deficit for which there isstill no effective clinical treatment. Tissue engineering of skeletalmuscle in vitro for functional tissue replacement in vivo may provide apotential therapeutic solution to this unmet medical need. In fact,significant progress has been made during last 15 years in understandingsome of the basic requirements for creating tissue engineered skeletalmuscle constructs in vitro. Early studies necessarily focused mainly onthe production of highly differentiated muscle constructs andcharacterizing their properties in terms of response to stretch andother mechanical stimulation in a 2-D tissue culture system(Vandenburgh, Mechanical forces and their second messengers instimulating cell growth in vitro. Am J. Physiol. 262(3 Pt 2):R350-5(March 1992); Mechanical stimulation of skeletal muscle generateslipid-related second messengers by phospholipase activation. J CellPhysiol. 155(1):63-71 (April 1993).

The majority of recent work on 3-D cultures of skeletal muscle myoblastshas been performed using gel-based matrix and mechanical strainers; asbiodegradable scaffolds are thought to possess too much of a developmentbarrier (both structural and nutritional) to clinical development.Recently, 3-D cultures of myoblasts have been successfully establishedand isometric contractile responses in these 3-D constructs, termedmyoids, were measured (Dennis R G, Kosnik P E. Excitability andisometric contractile properties of mammalian skeletal muscle constructsengineered in vitro. In Vitro Cell and Dev Biol Animal. 36:327-335(2000)). Additionally, fibrin-based gels were suggested as another novelmethod to engineer 3-D functional muscle tissue. The latter achievedmuscle structures of 100-500 μm diameter with measured maximal tetanicforce of 805.8±55 μN (Huang Y et al., Rapid formation of functionalmuscle in vitro using fibrin gels. J Appl Physiol 98: 706-713 (2005)).In short, tissue engineered 3-D skeletal muscle constructs composed ofcollagen or fibrin gels have clearly improved the understanding ofskeletal muscle organogenesis and provide a reasonable model forstudying the developmental physiology of skeletal musclemicro-structures in vitro.

However, while muscle constructs developed with synthetic scaffolds cansupport the contractile portion of the muscle tissue, and furthermore,can be maintained in culture for several months, this approach still hassignificant limitations for clinical utility. For example, implantationof tissue engineered skeletal muscle constructs will require that theybe of relevant size and mechanical strength to be amenable to the rigorsof the requisite surgical procedures. Clearly, gel-based constructs arecurrently too small and too fragile for such surgical manipulatation.

As such, one of the major barriers to engineering clinically applicablefunctional muscle tissues for reconstructive procedures is the lack of abioreactor system and methodology that would accelerate cellularorganization, tissue formation and function.

SUMMARY OF THE INVENTION

A first aspect of the invention is a method of culturing organizedskeletal muscle tissue from precursor muscle cells. In general themethod comprises: (a) providing precursor muscle cells on a support in atissue media; then (b) cyclically stretching and relaxing the support atleast twice along a first axis during a first time period; and then (c)optionally but preferably maintaining the support in a substantiallystatic position during a second time period; and then (d) repeatingsteps (b) and (c) for a number of times sufficient to enhance thefunctionality of the muscle tissue (e.g., its ability to contract), orproduce organized skeletal muscle tissue, on the solid support from theprecursor muscle cells.

An alternate embodiment of the foregoing includes the step of cyclicallystretching and relaxing said support at least twice along a second axisduring said first time period (with stretching and relaxing along stilladditional axes being possible if desired).

A second aspect of the invention is cultured skeletal muscle tissueproduced by a process as described herein.

A third aspect of the invention is cultured skeletal muscle tissue. Thetissue is characterized by cells that exhibit, or its ability toexhibit, a reproducible contractile response to KCl-induceddepolarization in vitro. In some embodiments the tissue is furthercharacterized by a unidirectional orientation on histologicalexamination; the presence of multinucleated myofibril cells; cells thatexpress muscle markers as confirmed by immunohistochemistry (e.g., alphaactin and myosin heavy chain); contains and cells that produceextracellular matrices as confirmed by Mason's Trichrome.

A further aspect of the invention is a device useful for carrying out amethod as described herein. The device preferably comprises a container,a pair of engaging members in said container for engaging tissuesupports or other tissue constructs, an actuator mounted on thecontainer and operatively associated with one of said engaging membersto provide controlled cyclic strain to attached tissue supports or othertissue constructs, a motor connected to said actuator, and a controlleroperatively associated with said motor. All are positioned so thatsupports or constructs carried by the engaging members may be immersedin a suitable growth or culture media in the container. The controlleris configured to implement a method as described herein. The engagingmembers preferably include a plurality of points of attachment so that aplurality of tissue supports or tissue constructs, each with its ownvolume of space within the container, are provided.

The foregoing and other objects and aspects of the present invention areexplained in greater detail in the drawings herein and the specificationset forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an apparatus of the present invention.

FIG. 2 is a top view of an apparatus of the present invention. Thebioreactor engineered tissue is cyclically stretched by a linear motorthat is, in turn, connected to a computer control system. There is greatflexibility in the equipment and computer software for satisfying thedesired biological boundary conditions.

FIG. 3. H&E staining of bioengineered skeletal muscle following 5 daysin the bioreactor. Note the dramatic change in orientation of themyocytes during static (A) growth versus culturing in the bioreactor(B). (C) Representative tracing of the contractile response ofbioengineered muscle to KCl-induced depolarization in organ bathstudies. A similar response was observed on two other strips. Myocytesseeded on a static scaffold in the same incubator for the same timeperiod, exhibited no detectable contractile response to addition of KCl.

FIG. 4 shows staining of control skeletal muscle (A) and skeletal musclecultured 7 days (B) in a bioreactor in accordance with methods of thepresent invention.

FIG. 5. Panels A & B provide representative examples of the contractileresponses to electrical field stimulation (EFS) observed 2 and 4 weeksafter implantation of bioengineered skeletal muscle on the latissimusdorsi of nu/nu mice.

Panel C is normal muscle as a control.

FIG. 6. Graphical summary of the results of physiological experimentsperformed at the 4 week time point on isolate skeletal muscle tissuestrips in vitro.

FIG. 7. Graphical summary of the results of all physiologicalexperiments performed on isolate skeletal muscle tissue strips in vitro.

The present invention is explained in greater detail in the drawingsherein and the specification below. The disclosures of all United Statespatent references cited herein are to be incorporated by referenceherein in their entirety.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Subjects to be treated by the methods of the present include both humansubjects and other animal subjects (particularly mammalian subjects suchas dogs and cats) for veterinary purposes.

Muscle cells used to carry out the present invention are preferablymammalian muscle cells, including primate muscle cells, including butnot limited to human, pig, goat, horse, mouse, rat, monkey, baboon, etc.In general such cells are skeletal muscle cells. Muscle cells of otherspecies, including birds, fish, reptiles, and amphibians, may also beused if so desired. The muscle cells are, in general, precursor cells,or cells that are capable of differentiating into muscle cells,specifically skeletal muscle cells, under appropriate culture conditionsand stimuli as described herein. Muscle precursor cells are known. See,e.g., U.S. Pat. No. 6,592,623.

“Supports” on which muscle cells may be seeded and grown to producecultured muscle tissue of the present invention include any suitablesupport. See, e.g., U.S. Pat. Nos. 6,998,418; 6,485,723; 6,206,931;6,051,750; and 5,573,784. Collagen supports or decellularized tissuesupports (e.g., obtained from smooth muscle or skeletal muscle, such asa decellularized mammalian (e.g., porcine) bladder, are currentlypreferred.

Any suitable culture media can be used to grow cells in the presentinvention, including medias comprising serum and other undefinedconstituents, defined medias, or combinations thereof, such as RPMI,DMEM, etc. If desired an angiogenic compound such as VEGF can beincluded in the media to facilitate the formation of vascular cells orvasculature in the muscle tissue.

A device of the present invention is schematically illustrated inFIG. 1. The device comprises an actuator mounted on a tissueculture-compatible container and configured to provide controlled cyclicstrain to attached scaffolds or other tissue constructs 21. Theseconstructs are normally stretched in one direction. Constructs aresupported between two holders or support engaging members 10 and 11,which can be formed from any structure for attaching a scaffold ordeveloped tissue for tissue culture. The device has a removable lid, 12,which is a necessary component for setting up tissues for culturing andnecessary to cover the box fully during culturing. The device has asingle pass-through 13 consisting of a water-resistant bearing andmounting hardware. The device contains vertical adjustments both on theactuator connecting piece 14 (facilitated by a vertical slot and bolt,not shown) and on the pieces on the opposite side of the box 15 (alsofacilitated by vertical slot and bolt, not shown). The linear actuatorstator (17) is controlled by a linear motor controller system (18) and acontrol computer (19), used for programming and subsequent monitoring ofthe actuator. The shaft (16) gives the motion to the support connectingpiece 14, moving the horizontal piece 10 which provides the cyclicdeformation to the constructs 21. The crosspieces 10, 11 can have anynumber of connecting points 22 for constructs. The connecting points arespaced so that each construct has its own volume immersed (fully orpartially) in the culture medium within the apparatus.

While the device is shown with a single motor and actuator, it will beappreciated that an additional motor and engaging member may also beincluded to provide for elongation and relaxation along a second axis,if desired.

The apparatus may be used in a method of culturing organized skeletalmuscle tissue from precursor muscle cells. As noted above, the methodcomprises: (a) providing precursor muscle cells on a support (e.g., acollagen support) in a tissue media; then (b) cyclically stretching andrelaxing the support at least two or three times, up to 5, 10 or 20times or more, along a first axis or direction of travel during a firsttime period. A preferred embodiment comprises (c) maintaining thesupport in a substantially static position during a second time period;and then (d) repeating steps (b) and (c) for a number of timessufficient to produce organized skeletal muscle tissue on the solidsupport from the precursor muscle cells.

If desired an angiogenic compound such as VEGF can be seeded on orcarried by the solid support to facilitate the formation of vascularcells or vasculature in the muscle tissue.

The length of stretching of the solid support may be to a dimension atleast 5% greater in length than the static position, and the relaxingmay comprise retracting the support to a dimension not greater in lengththan the static position. In some embodiments the “static position” maybe intermediate between the stretched and relaxed position, and in suchcases the relaxing may comprise retracting the support to a dimension atleast 5% lesser in length than the static position.

The first time period, during which the stretching and relaxing occurs,may be of any suitable length, for example from 2 or 3 minutes up to 10,20 or 30 minutes in duration or more.

The second time period during which the support is maintained in astatic position, may be of any suitable duration. In some embodimentsthe second time period is shorter than the first time period, and may befrom 1 or 2 minutes in duration up to 10 or 20 minutes in duration. Inother embodiments the second time period is longer than the first timeperiod, and may be from 10 or 20 minutes in duration up to 40, 60 or 90minutes in duration, or more. In some embodiments, such as where thefirst time period contains comparatively long intervals betweenstretching and relaxing, the need for a second time period may beobviated altogether.

In one preferred embodiment, the support is cyclically stretched andrelaxed during a first “active” time period to a dimension of 10 percentgreater and lesser in length than the static dimension at a rate of 3cycles per minute for a total of five minutes, followed by a 55 minute“rest” second time period, continuously for 1 to 3 weeks of in vitroculture.

A particular advantage and application of the present invention is itsability to speed, accelerate or enhance the functional maturation orperformance of muscle such as skeletal muscle grown in vitro (e.g., asexhibited by the ability of the muscle tissue to contract in response tocontact to a 60 milliMolar KCl solution in vitro). Thus in someembodiments the total culturing time of the tissue, such as therepeating of steps (b) and (c) is carried out for a time of up to fivedays, or a time of up to one, two or three weeks, after which time acontractile response is preferably observed, with shorter culture timesbeing preferred.

Skeletal muscle tissue produced as described herein may be used invitro, in the apparatus described herein or in a separate apparatus, toexamine the pharmacological or toxicological properties of compounds ofinterest (e.g., by adding the compound of interest to a culture mediumin which the tissue is immersed, and examining the histological ormechanical properties of the tissue as compared to a control tissue).

Skeletal muscle tissue (with or without support) produced by the methodsof the present invention is preferably “suturable” in that it hassufficient structural integrity to be surgically sutured or otherwisefastened at either end when implanted and thereafter develop tensionupon contraction.

Skeletal muscle tissue produced as described herein may be used for thereconstruction of damaged tissue in a patient, e.g., a patient with atraumatic injury of an arm or leg. Such tissue may be utilized on thesupport (which is also implanted) or removed from the support andimplanted into the subject. The skeletal muscle tissue may be implantedto “build” soft tissue (e.g., at the interface between an amputated limband a prosthetic device) or to reconstruct (partially or totally) adamaged muscle (e.g., a muscle of the face, hand, foot, arm, leg, backor trunk). The cultured skeletal muscle tissue preferably has, in someembodiments, a size or volume of at least 1, 2, or 3 or more cubiccentimeters (not counting the volume of the support if present), and/ora length of 1 cm to 50 cm, to provide sufficient tissue mass forimplantation in a patient (e.g., in association with an existing muscleof the patient) and reconstruction of a skeletal muscle involved in, forexample, movement of fingers.

For allogenic transplant into a patient, skeletal muscle as describedherein may be matched or tissue-typed in accordance with knowntechniques, and/or the subject may be administered immune suppressiveagents to combat tissue transplant rejection, also in accordance withknown techniques.

The present invention is explained in greater detail in the followingnon-limiting Examples.

EXAMPLE 1

A bioreactor system consisted of an actuator mounted on a tissueculture-compatible container which was designed to provide controlledcyclic strain to muscle tissue scaffolds, as shown in FIG. 2. The linearactuator was controlled by a linear motor controller system and acontrol computer, used for programming and subsequent monitoring of theactuator. Primary human skeletal muscle precursor cells were isolated,grown and expanded in culture. The cells were seeded onto collagen-basedmuscle scaffold strips derived from porcine bladder tissue (1.0×0.3×0.3cm³). After two days of static culture, the muscle cell seeded scaffoldswere placed in the bioreactor system and programmed linear stretchingcycles were applied (LinMot®). The controlled cycle strain wasprogrammed to exert ±10% of the initial length of the cell seededscaffolds at a frequency of 3 times per minute for the first 5 minutesof every hour. The bioreactor was continuously operated for up to 3weeks after the initial set up. Muscle cell seeded scaffolds withoutcyclic stimulation served as controls. The muscle cell constructs wereassessed for structural and functional parameters using scanningelectron microscopy, histo- and immunohistochemistry, and physiologictissue bath studies.

As shown in FIGS. 3A, 3B, and 3C, the bioreactor engineered muscleproduced viable tissue with appropriate cellular organization. Scanningelectron microscopy of the bioreactor stimulated muscle tissue showed auniform attachment of muscle cells on the scaffold surface.Histologically, the bioreactor stimulated engineered muscle demonstratedunidirectional orientation by 5 days and continued to mature with timePresence of multinucleated myfibrils was evident within the tissueconstruct. The cells expressed muscle markers and produced extracellularmatrices over time, as confirmed by immunohistochemistry and by Mason'sTrichrome, respectively. The control scaffolds (myoblasts incubatedunder identical incubator conditions, but not in the bioreactor) showeddisorganized muscle cells without any directional orientation.Physiologic organ bath studies of the bioreactor applied engineeredmuscle exhibited a reproducible contractile response to KCl-induceddepolarization (p<0.05). The control scaffolds without the bioreactorstimulation failed to show any detectable contractile response.

This study demonstrates that an organized functional muscle tissue canbe engineered using a unidirectional tissue bioreactor system. Musclecell seeded scaffolds that are exposed to a constant cyclicbiomechanical stimulation are able to achieve enhanced cellularorganization and demonstrate significant contractile function. The useof this bioreactor system allowed for an enhanced cellular orientationand may accelerate muscle tissue formation for the bioengineering ofclinically relevant sized muscle tissues.

EXAMPLE 2

This example presents the results of more detailed studies with thebioreactor system described above. The system, in overview, consisted ofa linear actuator mounted on a tissue container to provide a controlledcyclic strain to muscle tissue scaffolds. Primary human muscle cellswere seeded onto scaffolds and placed in the bioreactor system andsubjected to cyclic strain equivalent to ≈±10% stretch of the originalscaffold length; strain was applied 3 times/min for the first 5 min/hourfor periods ranging from 5 days to 3 weeks. Following this conditioningprotocol, the cell constructs were assessed for structural andfunctional parameters in vitro; cell and scaffold constructs understatic culture conditions (i.e., no cyclic strain) were run in parallel.In a separate in vivo experiments, both the structures conditioned inthe bioreactors for 5 days and control tissue structures maintainedunder static culture conditions were implanted onto the latissimus dorsimuscle of nude mice. At 3, 5, and 7 days after implantation, structureswere retrieved and assessed for structural characteristics, while at 1,2, 3 and 4 weeks, functional parameters were assessed.

Materials and Methods

Preparation of acellular tissue matrices. Acellular tissue matrices wereprepared from porcine bladder as previously described (Ref). Briefly,excised porcine bladder tissues were placed in Triton X 1%(Sigma-Aldrich, Taufkirchen, Germany) for 24-48 h in the presence of0.1% sodium azide, while agitated in a water bath at 37° C. Theextraction of all cellular elements was confirmed histologically.Prepared acellular matrix was cut into the 1.5 cm×1.5 cm. Additionallongitudinal incisions were created to increase the surface area forcell seeding and sutures were placed at both ends in order to secure thescaffold in the bioreactor. Scaffolds were sterilized by soaking them inBetadine solution for 1 day, and subsequently washed with 1% antibioticPBC solution for 5 days before use in the these experiments.

Cell isolation, culture and characterisation. Primary human skeletalmuscles cells were isolated by surgical biopsy from (i.e., psoas muscle)healthy volunteers ages 25-35 under the guidelines of InstitutionalClinical Review Board of Wake Forest University Health Sciences Schoolof Medicine. Muscles were washed 3-4 times with sterile PBS to removedebris before being cut into small pieces. Muscle tissues were platedonto 35 mm culture dishes with myogenic medium [340 mL low glucose DMEM(GIBCO Life Science, catalog no. CC-3161), 100 mL FBS (Fetal BovineSerum), 50 mL HS (Horse Serum), 5 mL CEE (Chicken Embryo Extract) and 5mL Penicillin/Streptomycin. When cells had achieved confluence they werefurther expanded on 150 mm culture dishes. Cells were passaged atconfluence and always used before P10. Using this methodology weobserved that ≈75-85% of the cells were desmin positive, confirmingtheir myogenic phenotyped. P5 to P10 cells were transferred and seededon the surfaces of acellular scaffold (with dimensions of ≈1.5 cm³). Thecell seeded scaffolds were then incubated in DMEM for 24 h.

Mechanical strain. A linear motor-driven stimulator device (Linmot, Va.Tech) was used for applying the mechanical stimulation, which consistedof cyclic unidirectional stretch and relaxation. The bioreactor systemitself consisted of an actuator mounted on a tissue culture container inwhich the cell seeded scaffolds were secured. The linear actuator was,in turn, calibrated, controlled and programmed by a computer. To permitapplication of the cyclic stretch protocol, one end of the cell-seededscaffold (i.e., tissue construct) was tied via sutures on a stationarybar, while the other end was secured to the movable bar that wasattached to the linear motor and computer controller. As currentlydesigned, the container can hold up to 10 tissue constructs at one time,with the maximal distance between the two bars of ≈10 cm. The media werechanged every 3 days and tissue constructs were continuously providedwith 95% air-5% CO₂, at 37° C. in an incubator.

Cyclic strain protocol. In this study, primary human skeletal musclecells-seeded scaffolds were subjected to stretch and relaxation of ≈10%of their initial basal length. The exact protocol was as follows: tissueconstructs were stretched 3 times/min (i.e., the entire stretch andrelaxation protocol took 20 s) for the first 5 minutes of every hour forperiods ranging from 5 days to 3 weeks. Muscle cell seeded scaffoldswithout cyclic stimulation were placed in the incubator on 150 mmculture dishes and served as controls.

Experimental Design for in vitro studies. Primary cultured myoblasts(600×10⁶ cells/cm³) were seeded onto the collagen-based scaffold(1.5×0.3×0.3 cm³) derived from porcine bladder tissue. After 2 days ofstatic culture, the cell-seeded scaffolds were placed in the bioreactorsystem described above. After periods ranging from 5 days to 3 weeks ofmechanical stimulation, the muscle cell constructs were removed from thebioreactor system and assessed for structural and functional parameters.Again, muscle cell seeded scaffolds without cyclic stimulation served ascontrols.

Experimental Design for in vivo studies. Primary cultured myoblasts(600×10⁶ cells/cm³) were seeded onto the collagen-based scaffold(1.5×0.3×0.3 cm³) derived from porcine bladder tissue (see Methodsabove). After 2 days of static culture, the cell-seeded scaffolds weremoved into the bioreactor system. Following 1 week of mechanicalstimulation, the muscle cell constructs and control tissue constructswithout cyclic stimulation were implanted onto the lattissimus dorsimuscle of the nude mice). At 3, 5, and 7 days after implantation, theconstructs were harvested and were assessed for structural andhistological characteristics. At 1, 2, 3 and 4 weeks after implantationin vivo, the constructs were harvested and the contractility of thetissue constructs was assessed and evaluat to normal lattissimus dorsimuscle of the same nude mice.

Contractility Test. The KCl-induced contractile response was examinedfollowing 3 weeks of bioreactor conditioning in vitro. Contractilitytesting was also performed following 1 week of bioreactorpreconditioning and then after 7, 14, 21 and 28 post-implantation in thelatissimus dorsi of nude mide (5 tissue constructs/group). Thecontractile responses of the bioreactor-preconditioned muscle tissue wascompared to that of statically seeded bioengineered tissue (i.e., 1 weekof in vitro cell culture with no bioreactor preconditioning, as well asto the contractile responses observed in normal lattissimus dorsi muscleof similar dimensions that was harvested from the same nude mouse. Theprocedures for electrical field stimulation (i.e., EFS) followedpreviously published methods (5,6) with extensor digitorum longus (EDL)muscle (Radnoti Glass Technology Inc, Monrovia, Calif.).

After harvesting of the individual tissue constructs, the existingsuture was used to attach the one end of the construct to a forcetransducer (Radnoti Model TRN001, Monrovia, Calif.) that was mounted onthe spindle of a non-rotating micrometer head and then connected to anamplifier. The other end was attached to the glass hook at the bottom ofthe field-stimulating electrode (Radnoti model 160151). Thisconfiguration resulted in a vertically oriented muscle suspended betweenthe two parallel platinum electrodes. The entire preparation was thensubmerged in a 25-mL organ chamber (Radnoti model 158326) filled withKrebs solution of the following composition: (pH 7.4; concentration inmM: 122.0 NaCl, 4.7 KCl, 1.2 MgCl₂, 2.5 CaCl₂, 15.4 NaHCO₃, 1.2 KH₂PO₄,and 5.5 glucose). The solution was aerated with a 95% O₂-5% CO₂ gaseousmixture and maintained at 37° C. with the help of a polystat circulator(Cole-Parner Instruments, Chicago, Ill.), and changed at 15-minintervals. After a 10-minute period of temperature equilibration, theoptimal muscle length (i.e., Lmax) was determined by adjusting thestretch of the muscle through movement of the micrometer head. Afterdetermining the Lmax, isometric maximal twitch force was determined bygradually increasing voltage at 10 mV increments up to 100 mV, untilmaximal twitch force was achieved and recorded. Tetanic contractileforce was then measured at frequencies of 40, 70, 100, and 120 Hz, with1.5 s for each stimulation. The voltage used for tetanic testing was thesame used to create maximal twitch force, with a duration of 2 ms and adelay of 2 ms. A 3-min rest period followed each stimulation. Electricalstimulation was provided by a neurostimulator (model S44B, GrassInstruments, Quincy, Mass.) and delivered to the constructs throughplatinum electrodes. Data were recorded and stored using a computerizeddata acquisition software (Mac Lab hardware and software; ADIInstruments, Natick, Mass.). At the conclusion of the contractilemeasurements, all muscles were weighed. All force measurements wereobserved on a digital display and recorded on a chart recorder.

Results: Compared to control structures under static culture conditions,structures derived from the bioreactor conditioning protocol (i.e.,engineered tissue) produced viable muscle tissue with appropriatecellular organization. The engineered muscle showed unidirectionalorientation within 5 days of bioreactor conditioning, and continued tomature with time. The presence of organized myofibrils was evident withthe expression of muscle markers in the bioreactor stimulatedstructures. Extending the bioreactor conditioning period to 3 weeksproduced a bioengineered tissue capable of generating a contractileresponse to depolarization with KCl. Finally, implantation of theengineered muscle tissue into the latissimus dorsi of nude micefollowing 3 days to 4 weeks of bioreactor conditioning yielded tissueswith numerous structural and histological similarities to skeletalmuscle, and 4 weeks after implantation the bioengineered muscle tissueshowed a reproducible contractile response to EFS (p<0.05) that wasapproximately 30-50% of the response observed on comparably sizedcontrol segments from the same animal. No detectable contractileresponses were observed on statically seeded constructs at any timepoint studied following implantation. In addition, in all cases,scaffolds maintained under static culture conditions showed disorganizedtissue formation and multidirectional orientation of muscle cells bothin vitro and in vivo.

FIG. 4 shows staining of control skeletal muscle (A) and skeletal musclecultured 7 days (B) in a bioreactor in accordance with methods of thepresent invention. Note the unidirectional orientation of culturedmuscle (B) as compared to control muscle (A).

FIG. 5, panels A and B provide representative examples of thecontractile responses to electrical field stimulation (EFS) observed 2and 4 weeks after implantation of bioengineered skeletal muscle on thelatissimus dorsi of nu/nu mice. Compare these responses with thatobserved in native latissimus dorsi muscle from an nu/nu mouse. Of majorimportance, only one month after implantation of bioengineered muscle,we observed 30-50% of the contractile response produced in nativeskeletal muscle.

FIG. 6 provides a graphical summary of the results of physiologicalexperiments performed at the 4 week time point on isolate skeletalmuscle tissue strips in vitro. Where: Normal denotes experimentalresults obtained with native skeletal muscle; Active denotes retrievedbioreactor preconditioned bioengineered skeletal muscle (i.e., active) 4weeks after implantation on the latissimus dorsi; Control denotesexperimental results obtained on bioengineered skeletal muscle that wasNOT preconditioned in the bioreactor (cells+scaffold only and kept in anincubator under static conditions prior to implantation). See Methodsfor details. As illustrated, although the normal tissue produced greatercontractile responses under all experimental conditions studied, thebioreactor preconditioned bioengineered skeletal muscle was able togenerate contractile responses that ranged from ≈30-50% of normalmuscle. In stark contrast, in the absence of bioreactor preconditioning(i.e., Control), no detectable contractile responses were observed at 4weeks (or any other time point). Actual mean±SEM values can be found inthe corresponding Table 1.

TABLE 1 Changes of Contractility at 50 V Normal Active (4 weeks) ControlNo. Animal 5 5 5  40 Hz 4.09 ± 1.59 2.18 ± 0.98 0  70 Hz 9.55 ± 2.042.78 ± 1.12 0 100 Hz 15.49 ± 4.25  4.66 ± 1.37 0 120 Hz 16.8 ± 4.09 5.40± 1.52 0 Max twitch 9.16 ± 2.12 3.46 ± 1.05 0 (100 V) (90 V) Data areexpressed as Mean and SD.

FIG. 7 provides a graphical summary of the results of all physiologicalexperiments performed on isolate skeletal muscle tissue strips in vitro.As illustrated, within 2 weeks after implantation, bioreactorpreconditioned bioengineered skeletal muscle strips are capable ofgenerating measurable contractile responses. Again, while thebioengineered skeletal muscle generated significantly less tension thannative latissimus dorsi at all time points thus far studied, these arethe first data that we are aware of which document force generation ofthis magnitude in bioengineered human skeletal muscle. Actual mean±SEMvalues can be found in the corresponding Table 2.

TABLE 2 Time-dependent changes in contractility ofbioreactor-conditioned engineered muscle tissue Normal 1 wk. 2 wks. 3wks. 4 wks. No. Animal 5 5 5 5 5  40 Hz 4.09 ± 1.59 0 0.32 ± 0.13 0.19 ±0.1  2.18 ± 1.31  70 Hz 9.55 ± 2.04 0 0.67 ± 0.63 1.73 ± 0.72 2.78 ±1.12 100 Hz 15.49 ± 4.25  0 1.75 ± 1.31 4.21 ± 1.70 4.66 ± 1.47 120 Hz16.8 ± 4.09 0 2.83 ± 1.42 3.64 ± 1.89 5.40 ± 1.53 Data are expressed asMean and SD. There was no contractile response in all control (cells,but not bioreactor conditioning) constructs of 1 wk, 2 wks, 3 wks and 4wks.

Conclusion: This study demonstrates that organized functional muscle canbe engineered using a computerized bioreactor system on a biodegrablescaffold (matrix). That is, following isolation and expansion, musclecell seeded scaffolds that are exposed to a cyclic stimulation protocolare able to achieve enhanced cellular organization and acceleratedtissue formation/maturation both in vitro and in vivo, with significantcontractile function in all cases. The use of this bioreactor system mayaccelerate muscle formation for reconstructive or replacement surgery inpatients with localized functional skeletal muscle deficits.

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The foregoing is illustrative of the present invention, and is not to beconstrued as limiting thereof. The invention is defined by the followingclaims, with equivalents of the claims to be included therein.

That which is claimed is:
 1. Cultured organized skeletal muscle tissueproduced by a process comprising: (a) providing a decellularized tissuesupport in a tissue media in a bioreactor, said support comprisingprecursor muscle cells; then (b) cyclically stretching and relaxing saidsupport at least twice along a first axis during a first time period;and then (c) maintaining said support in a substantially static positionduring a second time period; and then (d) repeating steps (b) and (c)for a number of times sufficient to produce said cultured organizedskeletal muscle tissue on said support from said precursor muscle cells,wherein repeating steps (b) and (c) is carried out for a time of up totwo weeks and results in the cultured organized skeletal muscle tissuehaving unidirectional orientation, and wherein said cultured organizedskeletal muscle tissue is further characterized by enhancing cellularorganization, contractile function and tissue formation when implantedin vivo.
 2. The cultured organized skeletal muscle tissue of claim 1,wherein said cyclically stretching and relaxing is carried out at leastthree times during said first time period.
 3. The cultured organizedskeletal muscle tissue of claim 1, wherein said stretching comprisesextending said support to a dimension at least 5% greater in length thansaid static position.
 4. The cultured organized skeletal muscle tissueof claim 1, wherein said first time period is from 2 to 30 minutes induration.
 5. The cultured organized skeletal muscle tissue of claim 1,wherein said second time period is from 10 to 100 minutes in duration.6. The cultured organized skeletal muscle tissue of claim 1, whereinsaid repeating of steps (b) and (c) is carried out for a time of fivedays to one week.
 7. The cultured organized skeletal muscle tissue ofclaim 1, wherein said precursor muscle cells are human cells.
 8. Thecultured organized skeletal muscle tissue of claim 1, wherein saidcultured organized skeletal muscle tissue is suturable and is 1 to 50 cmin length.
 9. The cultured organized skeletal muscle tissue of claim 1,wherein said cultured organized skeletal muscle tissue contracts inresponse to electrical stimulation four weeks after in vivoimplantation.
 10. The cultured organized skeletal muscle tissue of claim1, wherein said decellularized tissue support comprises decellularizedsmooth muscle or decellularized skeletal muscle.
 11. The culturedorganized skeletal muscle tissue of claim 10, wherein saiddecellularized tissue support comprises decellularized mammalianbladder.
 12. An implantable skeletal muscle construct produced by aprocess comprising: (a) providing a suturable support in a tissue mediain a bioreactor, wherein said suturable support comprises mammalianprecursor muscle cells, and wherein said suturable support comprisesdecellularized smooth muscle or decellularized skeletal muscle; then (b)cyclically stretching and relaxing said suturable support at least twicealong a first axis during a time period of from 2 to 30 minutes induration; and then (c) maintaining said suturable support in asubstantially static position for a time period of from 10 to 100minutes in duration; and then (d) repeating steps (b) and (c) for anumber of times sufficient to produce said implantable skeletal muscleconstruct on said suturable support from said precursor muscle cells,wherein repeating steps (b) and (c) is carried out for a time of up totwo weeks and results in the implantable skeletal muscle constructhaving unidirectional orientation, and wherein said implantable skeletalmuscle construct is further characterized by enhancing cellularorganization, contractile function and tissue formation when implantedin vivo.
 13. The implantable skeletal muscle construct of claim 12,wherein said suturable support comprises decellularized mammalianbladder.
 14. The implantable skeletal muscle construct of claim 12,wherein said suturable support comprises decellularized porcine bladder.15. The implantable skeletal muscle construct of claim 12, wherein saidprecursor muscle cells are human cells.